A COMPARISON OF SIMULATED PRECIPITATION BY HYBRID ISENTROPIC-SIGMA AND SIGMA-MODELS

The primary objectives of this study are threefold: 1) to compare simu lations of dry and moist baroclinic development from 10- and 22-layer hybrid isentropic-sigma coordinate models with those from 11-, 27-, an d 35-layer sigma coordinate models; 2) to examine the ability of the m odels to transport water vapor and simulate equivalent potential tempe rature theta(e); and 3) to compare predictions of the timing, location , and amount of precipitation. A model's capability to predict precipi tation stems from the accuracy of its simulation of the joint distribu tion of mass, potential temperature, and water vapor throughout the mo del domain. In a series of experiments to compare simulations of preci pitation, several analytic distributions of water vapor are specified initially. The water vapor distributions include a ''cylinder'' extend ing vertically throughout the atmosphere and ''lenses'' within isentro pic, sigma, and isobaric layers. The effects of increased horizontal r esolution are also studied. Results indicate that when the relative hu midity is vertically uniform through a substantial extent of the atmos phere, all the models produce very similar precipitation distributions . However, when water vapor is confined to relatively shallow layers, the ability of the sigma coordinate models to simulate the timing, loc ation, and amount of precipitation is severely compromised. Furthermor e, the 10-layer hybrid model conserves theta(e) to a higher degree of accuracy and simulates a more realistic evolution of precipitation eve n when compared to results from sigma models with increased vertical a nd horizontal resolution. In all instances, the experiments demonstrat e that advantages reside in prediction of precipitation with the hybri d model. Both theoretical and conceptual bases for these differences a re provided.